Related Applications
This application is concurrently filed with U.S. patent Ser. No. 08/430,774, (TI-19475) titled "LOW COST PROGRESSIVE SCAN TELEVISION WITH SPECIAL FEATURES."
1. Field of the Invention
This invention relates to television systems with interlaced input, more particularly to television systems that convert interlaced input to progressive scanned data.
2. Background of the Invention
Most typical broadcast television standards consist of interlaced fields. A typical video frame is transmitted in two fields, the first field normally contains the odd numbered lines of the frame, the second contains the even numbered lines. Typical display systems use the incoming data for display on a cathode-ray tube, but other options are available for displaying the data.
Spatial light modulators typically consist of arrays of individual elements, each addressable by memory cells. The addressing is typically done along columns of the array. This makes the addressing necessary to accommodate interlaced data very difficult and often too slow. Progressively scanned pictures result in a better utilization of the modulators. Also, progressively scanned pictures result in better picture quality, especially with larger projection; size images as are available with spatial light modulators.
A special problem arises when the desired data to be converted to progressive scan is from film sources. Film is an inherently progressive, 24 MHz material. Systems operating in NTSC typically convert to 60 Hz interlace by means of a 3:2 pulldown as shown in FIG. 1 (prior art). The frame input arrives with complete frames as shown at frame boundaries 12 and 14. The first frame between boundaries 12 and 14 is scanned such that the even numbered lines go to frame 1, even, between boundaries 18 and 20, and the odd numbered lines go to frame 1, odd, between boundaries 20 and 22.
The even numbered lines from the first film frame also go to the even numbered lines for frame 2, even. Frame 2, odd, is made up of the odd numbered lines from the second film frame, between boundaries 14 and 16. The even numbered lines for frame 3 come from the second film frame. Frame 3, odd, would then be from the third film frame. As can be seen by this example, it is referred to as 3:2 pulldown, because every other frame contributes 3 fields to the interlaced input, and the offset frames contribute 2 field to the interlaced input. For 50 Hz sources, such as PAL, the film is merely run at 25 Hz and doubled to fill the 50 Hz interlaced requirement.
Some methods for converting back from the interlaced to progressively scanned sources include a means for detecting which field is from the 3-segment frame and which is from the 2-segment frame, by incorporating a code into the data driving the 3:2 pulldown operation. This code can then be read by the processor at the display end and it can use the appropriate fields to regain the original film data at 60 Hz. A second solution uses a processor to implement a pattern detector that detects the 3-2-3-2 pattern of 3:2 pulldown and marks the corresponding fields to recover the original film. This solutions are more completely discussed in U.S. Pat. Nos. 4,876,596 and 4,982,280 assigned to Faroudja, Inc. These approaches require either an explicit specification of 3:2 pulldown, or the ability to detect the pattern. A more general approach is needed that is general enough to work without requiring explicit specification at the transmission end or detection of the 3-2-3-2 pattern.
Additionally, one problem with these solutions lies in the area of motion detection. Incorrect motion detection leads to visual artifacts, such as tearing on moving areas, blurriness in the presence of motion due to line averaging, and blocky reconstruction of oriented edges. Current progressive scan processes can eliminate these problems, but require extra processing time and more memory, leading to increased costs.